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J 


EXPERIMENTING 


WITH 

. 

Induction  Coils. 

Containing  practical  directions  for  operating  Induction  Coils 
and  Tesla  Coils ; also  showing  how  to  make  the 
apparatus  needed  for  the  numerous 
experiments  described. 


H.  S.  NORRIE, 

Author  of  “ Induction  Coils  and  Coil  Making.” 


Sixth  Thousand. 

NEW  YORK: 

SPON  & CHAMBERLAIN,  123-125  Liberty  Street. 

LONDON: 

E.  & F.  N.  SPON,  LIMITED,  57  HAYMARKET,  S.W. 

L 


1915 


■ 


Copyright,  1906, 

By  Spon  & Chamberlaih. 


CAMELOT  PRESS,  18-20  Oak  Street,  New  York,  U.  S.  A. 


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PREFACE. 

1 , 


The  Ruhmkorff  coil  is  probably  the  most  inter- 
esting piece  of  electrical  apparatus  that  can  be 
operated  at  small  cost  and  without  danger. 

To  the  student  and  the  spectator  alike,  the  vast 
majority  of  the  experiments  capable  of  being  per- 
formed with  it  are  sufficiently  fascinating  to  while 
away  many  winter  evenings. 

The  possessor  of  a small  coil  and  a few  pieces 
of  auxiliary  apparatus  which  are  easily  constructed 
has  a fund  from  which  to  draw  numberless  useful 
and  interesting  experiments. 

It  is  difficult  to  set  down  in  cold  black  and 
white  a graphic  description  of  the  many  beautiful 
color  and  spark  effects  that  can  be  produced 
within  even  a small  coil.  The  marvelous  changes 
of  color  and  vivid  lightning  flashes  of  the  high 
tension  spark  both  in  vacuum  and  in  the  air,  pre- 
clude description. 

From  the  fact  that  most  of  the  experiments 
are  spectacular,  it  is  not  feasible  to  describe  them 
at  length  without  the  aid  of  colored  plates.  And 
even  with  their  aid  but  a faint  conception  of  the 
colors  could  be  attempted. 


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2 


PREFACE. 


The  following  pages  therefore  will  be  devoted 
to  giving  such  practical  hints  on  coil  handling  and 
the  manufacture  of  auxiliary  apparatus,  together 
with  a number  of  notes  describing  the  lines  upon 
which  the  experimenter  may  follow  to  obtain  the 
best  results. 

No  data  are  given  of  the  actual  construction  of 
the  Ruhmkorff  coil  as  these  are  to  be  found  in 
books*  devoted  to  the  subject. 

It  cannot  be  too  strongly  emphasized  right 
here  that  the  experiments  with  the  high  tension 
currents  from  the  induction  coil  are  often  danger- 
ous if  directed  upon  a human  being;  and  no  at- 
tempt should  be  made  to  use  them  for  practical 
jokes.  The  consequence  in  the  latter  case  might 
be  serious,  depending  upon  the  strength  of  the 
coil  and  the  nervous  state  of  the  person  receiving 
the  current. 

In  medical  applications  of  high  tension  elec- 
tricity the  actual  current  is  rarely  applied  directly 
to  the  body,  but  rather  acts  by  induction  through 
large  open  spirals  in  the  centre  of  which  the  pa- 
tient is  located. 


*Norrie — Induction  Coils. 
Allsop — Induction  Coils . 


CONTENTS. 


Introduction. — The  kind  of  coil  available  ...  7 

CHAPTER  I. 

The  Handling  of  a Ruhmkorff  Coil. 

The  Ruhmkorff  coil — Dischargers — Battery  for  the 
coil — Handling  of  the  coil — Wires  and  their  in- 
sulation— Precautions  to  be  taken  in  working 
with  induction  coils — Experiments  with  static 
electricity  made  with  the  induction  coil — Con- 
densers and  the  charging  of  the  same — How  to 
make  a condenser — Induction  coils  on  electric 
light  circuits 9 


CHAPTER  II. 


Experiments  with  Sparks. 

Increasing  the  spark  length — Disruptive  effects — Po- 
larity of  terminals — Production  off  ozone — Ex- 
periments with  sparks — Condensers  and  their 
use. — The  electric  arc — Cohesion  designs — Spark 

photographs — Revolving  effects 24 

3 


4 


CONTENTS. 


CHAPTER  III. 

Effects  in  the  Vacuum. 

Fluorescence — Vacuum  tests — Geissler  tubes — A sim- 
ple rotator  for  Geissler  tubes — Colors  in  rarefied 
gases — Production  of  a vacuum — Mercury  vacu- 
um— Mercury  pumps — Cements  for  apparatus  to 
exclude  air 39 

CHAPTER  IV. 

Induction  and  Wireless  Telegraphy. 

Hertz  induction  apparatus — Resonator — Spiral  in- 
duction coil  for  induction  signaling — Wireless 
telegraphy — Tesla  coils — How  to  make  Tesla 
coils — Discharger — Oil  condenser — The  discharge 
from  a Tesla  coil 53 


1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 


10 

11 

11 

12 

15 

20 

22 

22 

24 

28 

30 

34 

37 

40 

42 

45 

48 

50 

54 

56 

58 

61 

62 

64 

66 

70 


LIST  OF  ILLUSTRATIONS, 


Discharger 

Adjustable  discharger 

Connected  discharger 

Gladstone-Lalande  cell 

Reversing  switch 

Charging  Leyden  jar 

Lining  jar  with  tin-foil 

Lining  jar  with  tin-foil 

Point  and  plate  discharge  . 

Decomposition  of  water 

Wandering  sparks 

Lead  pencil  arc 

Rotating  disc 

Vacuum  test  of  lamp 

Geissler  tubes 

Rotator  for  tubes 

Mercury  vacuum  tube 

Mercury  pump  for  vacuum  . 

Hertz  resonator 

Induction  spiral 

Wireless  telegraph  circuit 

Detail  of  Tesla  coil 

End  of  coil  frame 

Tesla  coil  wound 

Case  for  coil 

Connections  of  Tesla  coil 

5 


1 


INTRODUCTION. 


By  the  term  Ruhmkorff  coil  is  not  necessarily 
meant  the  elaborate  and  expensive  coils  made  by 
the  manufacturers  for  X-ray  work  or  for  wireless 
telegraphy. 

Although  coils  of  large  size  are  costly  and  con- 
tain all  kinds  of  devices  to  enable  the  best  results 
to  be  produced,  the  experimenter  may  attempt 
much  with  the  simple  coil  made  for  automobile 
work. 

The  manufacturers  of  the  latter  apparatus  mostly 
all  furnish  coils  of  this  description,  giving  a spark 
one  inch,  or  perhaps  two  inches,  in  length.  Such 
coils  can  be  obtained  at  but  little  more  expense 
than  those  regularly  made  for  gas  or  gasolene 
ignition. 

The  main  objection  to  them  is  that  the  contact 
breaker  is  not  capable  of  wide  adjustment.  Being 
generally  constructed  for  the  requirements  of  gas 
ignition,  it  works  at  a constant  speed. 

But  there  is  not  much  mechanical  ingenuity  re- 
quired in  fitting  one  of  these  coils  with  an  ad- 
justable contact  breaker  if  desired. 

And  most  of  the  experiments  to  be  described 
will  be  possible  with  the  ordinary  fast  and  con- 
stant speed  attachment. 


7 


CHAPTER  I. 


How  to  Handle  the  Coil. 

The  Ruhmkorff  Coil  consists  of  five  main  parts; 
the  primary  coil,  the  secondary  coil,  the  core,  the 
condenser  and  the  contact  breaker. 

The  primary  coil  has  generally  two  layers  of 
insulated  copper  wire  carrying  the  current  from 
the  battery,  or  other  source  of  current.  It  is 
wound  around  an  insulating  tube  which  surrounds 
the  core,  which  is  a bundle  of  fine  iron  wires  be- 
coming magnetized  or  demagnetized  according  as 
the  current  flows  through  the  primary  coil,  or  is 
interrupted  in  its  flow. 

The  secondary  coil  is  wound  on  an  insulation 
surrounding  the  primary  coil  and  is  of  many 
turns  of  extremely  fine  insulated  copper  wire. 
The  greater  the  number  of  turns  m the  secondary 
coil  as  compared  with  those  in  the  primary  coil 
the  greater  the  effect  from  the  secondary. 

The  condenser,  which  is  composed  of  layers  of 
sheets  of  tin-foil  and  insulation,  makes  the  core 
become  more  quickly  demagnetized.  The  quicker 
this  takes  place  the  more  effective  the  coil. 

The  contact  bleaker  is  an  apparatus  which  makes 


10 


THE  RUHMKORFF  COIL. 


and  breaks  the  battery  current  through  the  pri- 
mary coil. 

Discharger. — In  some  coils  the  ends  of  the  sec- 
ondary wire  are  brought  out  to  binding  posts 
provided  with  clamping  screws  for  the  attachment 
of  other  wires.  But  for  experimental  purposes  a 
discharger  mounted  across  the  coil  top,  or  on  its 
base,  is  preferable. 

A simple  form  of  discharger,  Fig.  1,  consists 
of  two  brass  pillars  P P to  which  are  fastened  the 
ends  of  the  secondary  coil. 


I ( 

f 

H 

P 

P H 

J 

Lass 

Fig.  1. 


Sliding  through  holes  in  these  pillars,  are  brass 
rods  provided  at  their  distant  ends  with  rubber 
handles  H — H. 

At  the  points  G wdiere  the  ends  form  an  air 
gap,  they  are  sometimes  pointed.  A better 
scheme  is  to  screw-thread  these  ends  so  that 
different  tips  can  be  attached  when  needed. 

Sliding  these  rods  in  and  out  of  the  pillars 
makes  the  air  gap  longer,  or  shorter,  as  desired. 

A better  discharger  is  shown  in  Fig.  2 where 
the  rods  slide  through  the  discs  D D which  turn 
in  slots  cut  in  the  top  of  the  pillars  P P . 


HOW  TO  HANDLE  THE  COIL. 


11 


In  this  case  the  air  gap  may  be  adjusted  by 
pushing  the  handles  down  or  up.  Also  in  some 
experiments  a plate  of  insulating  material  can  be 
placed  below  the  air  gap  and  the  spark  directed 
upon  it. 


G 


A more  complete  discharger  fitted  for  large 
X-ray  coils  is  shown  in  Fig.  3.  Here  the  dis- 
charger rods  are  free  to  move  up  and  down  as  in 
the  last  figure.  But  a connecting  rod  R is  so 


pivoted  to  each  discharging  rod  that  it  moves 
both  together. 

The  wheels  W W,  attached  to  the  pivots  of  the 
rods,  are  made  of  insulating  material.  Turning 
them  moves  both  rods  at  once 


12 


THE  RUHMKORFF  COIL. 


Battery  for  the  Coil.  The  selection  of  a battery 
depends  upon  the  size  of  the  coil  and  the  use  to 
which  it  is  to  be  put. 

For  a small  coil  of  an  inch  spark,  or  less,  six  or 
eight  large  dry-cells  will  answer  for  experiments 
of  a few  minutes’  duration.  But  dry  cells  soon 


Fig.  4. 


give  out  when  a large  current  is  taken  from  them. 
A wet  battery  is  preferable. 

The  various  Leclanche  cells,*  such  as  the  Samson, 
or  carbon  cylinder  cell,  are  not  much  better  than 

*See  Schneiders  “Modern  Primary  Batteries  ” for  full 
information  on  batteries  for  coils . 


HOW  TO  HANDLE  THE  COIL. 


13 


dry  cells,  as  they  do  not  furnish  a steady  current 
for  long  periods  of  use. 

The  acid  batteries  are  mostly  troublesome  to 
handle,  although  they  furnish  a large  current  for 
long  periods.  Two  Grenet  cells,  using  bichromate 
of  soda,  or  Electropoion  fluid,  are  sufficient  for  a 
small  coil. 

Storage  cells  are  above  all  others,  the  best  for 
large  coils,  but  require  recharging  from  a source 
of  direct  current. 

The  best  primary  battery  for  the  experimenter 
with  an  induction  coil  is  one  of  the  forms  of  copper 
oxide  cells  such  as  the  Gladstone-Lalande  (Fig.  4). 
This  cell  uses  caustic  soda  in  water  and  is  ex- 
tremely easy  to  handle. 

It  furnishes  a large  steady  current  and  main- 
tains its  full  strength  up  to  the  point  when  it 
needs  almost  entire  renewal  of  its  parts.  Four  to 
six  cells  are  sufficient  for  coils  giving  sparks  up  to 
two  inches  in  length. 

Handling  the  Coil. — It  is  useless  to  attempt  to 
insulate  the  wires  running  from  the  secondary 
terminals  with  ordinary  insulated  wire.  The  only 
means  of  insulation  is  to  keep  them  apart  by 
means  of  high  insulators,  or  to  hang  them  from 
silk  threads.  Glass  tubing  in  the  case  of  small 
coils  might  be  used  but  will  rarely  be  necessary. 
The  fine  copper  wire,  such  as  No.  30,  is  suitable 
for  connecting  apparatus  to  the  secondary  ter- 
minals but  should  not  be  wound  into  spirals. 


14 


THE  RUHMKORFF  COIL. 


It  must  be  remembered  that  the  remarks  as  to 
the  insulating  of  the  coil  terminal  wires,  also  ap- 
plies to  all  apparatus  connected  to  it. 

Large  metallic  objects  although  insulated  from 
the  earth,  or  one  terminal  of  the  coil,  will  lead  off 
the  current  by  acting  as  condensers,  if  allowed  to 
some  within  sparking  distance.  Dampness  is  also 
a deadly  enemy  of  insulation. 

Care  must  be  taken  to  avoid  shocks.  The  dis- 
charge from  even  a small  coil  is  painful,  and  under 
no  circumstances  should  a Ruhmkorff  coil  be  used 
in  playing  tricks  on  a person. 

The  effect  will  vary  of  course  with  the  size  of 
the  coil  and  the  nervous  condition  of  the  person 
experimented  on;  but  in  no  case  is  it  advisable 
to  allow  anybody  to  take  the  full  discharge  of  the 
coil. 

All  coils  should  have  a switch  in  the  battery 
circuit  that  can  be  instantly  opened,  when  neces- 
sary, so  that  the  coil  may  be  stopped  at  once. 

A reversing  switch  is  the  most  useful,  and  can 
be  made  out  of  a double-pole  double-throw  knife 
switch  as  shown  in  Fig.  5. 

In  most  experiments  it  is  necessary  that  the 
coil  and  all  the  apparatus  be  well  insulated  from 
each  other. 

Care  should  be  therefore  taken  that  the  table 
surface  is  bone  dry.  If  it  can  be  covered  with  a 
piece  of  rubber  sheeting,  or  be  well  varnished  with 
shellac,  so  much  the  better. 

High  tension  current  is  soon  dissipated  from 


HOW  TO  HANDLE  THE  COIL. 


15 


points,  or  sharp  edges,  as  can  well  be  seen  in  the 
dark  by  the  brush-like  discharges.  This  is  all 
lost  energy  and  is  to  be  avoided  as  far  as  possible. 

Binding  posts  and  terminals  should  be  rounded 
off  and  tin  foil,  unless  discharges  are  wanted, 
should  have  its  edges  rounded  off.  The  leakage 
therefore  must  be  looked  for,  not  only  through 
poor  insulation,  but  from  sharp  projections. 

Working  the  apparatus  in  absolute  darkness 
will  soon  show  where  the  leak  is,  and  its  remedy 


can  be  better  determined  in  the  dark  than  in  the 
light. 

Coils  on  Electric  Light  Circuits. — To  operate  a 
coil  on  a direct-current  electric  light  circuit,  a 
rheostat  is  necessary  to  cut  down  the  voltage  to 
that  needed  by  the  coil. 

At  the  same  time  the  sparking  at  the  contact 
breaker  will  be  so  violent  as  to  destroy  the  platinum 
contacts  in  a very  short  time. 

The  rheostat  and  the  coil  divide  the  total 


16 


THE  RUHMKORFF  COIL. 


voltage  of  the  electric  light  circuit  when  adjusted 
properly,  but  the  contact  breaker  breaks  the 
circuit  at  its  entire  voltage. 

On  alternating  current  circuits,  the  coil  may  be 
operated  by  screwing  down  the  contact  breaker 
so  as  to  short  circuit  it  and  detaching  the  con- 
denser. 

The  - resultant  spark  will  be  in  the  form  of  a 
flame  possessing  great  heating  qualities. 

The  induction  coil  operated  in  this  manner 
simply  becomes  a “ step-up  ” transformer  of  in- 
efficient design  and  cannot  well  be  run  continuously 
for  long  periods  on  account  of  various  heating 
effects.  For  the  best  results  a form  of  electrolytic 
interruptor  is  necessary. 

Electrolytic  Interruptors.  The  latter  consists  in 
one  type  of  two  porcelain  jars  and  two  sheet  lead 
electrodes  immersed  in  dilute  sulphuric  acid. 

One  jar  is  thick  and  acts  as  a containing  vessel 
for  the  solution,  the  second  jar  and  the  electrodes. 

The  inner  jar  is  very  thin,  being  usually  a 
porcelain  beaker  such  as  is  used  in  chemistry. 

If  the  outer  jar  is  6 inches  wide  by  8 inches 
high,  the  inner  jar  should  be  about  3 inches  wide 
by  8 inches  high. 

Four  small  holes  are  bored  through  the  sides 
of  the  inner  jar  as  near  the  bottom  as  possible, 
and  spaced  at  equal  distances  apart  round  the 
circumference. 

They  must  not  be  much  larger  than  will  freely 


HOW  TO  HANDLE  THE  COIL. 


17 


admit  of  the  passage  of  a pin.  Upon  the  correct 
relation  of  the  size  of  the  holes  to  the  amount  of 
current  to  pass  depends  the  success  of  the  interrupter. 
For  a coil  of  under  4 inches  spark  length  the  size 
given  is  best. 

Each  electrode  consists  of  a sheet  of  lead  about 
2 inches  wide  and  long  enough  to  fit  in  the  jars 
when  rolled  into  a ring. 

One  flat  lead  ring  goes  in  the  outer  jar,  fitting 
loosely  around  the  outside  of  the  inner  jar  and 
the  other  fits  loosely  inside  the  inner  jar. 

Lengths  of  thick  wire  solder  or  strips  of  sheet 
lead  are  soldered  one  to  each  lead  ring  for  con- 
nections and  should  be  long  enough  to  project 
for  several  inches  outside  of  the  jars. 

This  device  is  connected  in  series  with  the  coil 
so  that  the  electric  light  current  (either  direct  or 
alternating)  passes  through  it,  the  jars  first  having 
been  filled  with  sulphuric  acid  and  water  to  a 
sufficient  height  to  well  cover  the  lead  rings  in 
both  jars. 

The  proportions  of  acid  to  water  are  one  part 
of  acid  to  seven  of  water.  The  acid  is  always  added 
to  the  water  in  a thin  stream  stirring  the  solution 
as  it  is  mixed. 

Never  add  the  water  to  the  acid. 

It  is  a wise  precaution  to  interpose  a rheostat 
in  the  circuit  so  that  the  amount  of  current  may 
be  regulated. 

Upon  closing  the  circuit  the  solution  will  com- 
mence to  bubble  and  will  get  hot. 


18 


THE  RUHMKORFF  COIL. 


Bubbles  of  gas  will  form  in  the  holes  in  the 
inner  jar  and  stop  the  current  flowing  but  will 
be  dissipated  almost  as  soon  as  they  are  formed. 

This  action  causes  a flow  and  interruption  of 
the  current  far  superior  in  its  effect  upon  the 
coil  to  most  ordinary  mechanical  interruptors. 

This  apparatus  being  patented  cannot  be  made 
to  be  sold  except  upon  arrangement  with  the 
patentees. 

Electrolytic  interruptors  act  better  after  having 
worked  for  a few  minutes.  This  should  be  borne 
in  mind,  as  otherwise  the  experimenter  may 
think  there  is  trouble  with  the  apparatus. 

Coil  Operation.  It  is  an  unwise  plan  to  set  a 
coil  in  operation  unless  the  terminals  of  the 
secondary  be  near  the  sparking  distance  and  par- 
ticularly so  when  the  coil  is  being  operated  at  its 
limit,  that  is,  with  its  extreme  battery  power. 

The  tendency  then  is  for  the  high  tension  cur- 
rent to  discharge  somewhere,  and  it  may  do  so 
through  the  interior  insulation. 

It  is  also  unwise  to  work  a coil  at  a spark  length 
more  than  it  is  rated  to  give  by  using  excessive 
battery. 

Not  only  may  the  primary  coil  become  heated 
but  the  secondary  insulation  is  liable  to  become 
punctured.  In  this  event  the  coil  is  practically 
worthless  unless  it  be  so  constructed  that  a section 
or  two  of  the  secondary  can  be  removed  and  re- 
wound. 


HOW  TO  HANDLE  THE  COIL. 


19 


Violent  sparking  at  the  contact  breaker  will 
indicate  either  that  the  battery  is  excessive  or 
that  the  condenser  has  been  pierced,  or  become 
short  circuited. 

In  general,  all  the  experiments  described  in 
books  on  so-called  static  electricity  can  be  made 
with  an  induction  coil. 

It  must  be  remembered  however  that  the  cur- 
rent induced  in  the  secondary  coil  is  alternating. 
A current  at  break  of  vibrator  contact  is  in  op- 
posite direction  to  one  induced  at  the  make  of 
contact. 

But  as  the  induction  effect  is  greatest  at  the 
break,  the  simple  method  of  separating  the  spark 
points  permits  the  current  inducted  at  the  make 
of  contact  from  flowing  in  the  external  circuit. 

For  the  above  reason,  in  charging  a condenser 
or  Leyden  jar,  a spark  gap  must  always  be  pro- 
vided. 

To  perform  the  experiments  mentioned  above, 
a uni-directional  current  is  generally  required. 

This  is  best  obtained  by  first  charging  a con- 
denser, or  series  of  condensers,  and  using  their 
discharge  for  current  supply. 

Charging  Condenser.  The  actual  connections  for 
charging  a Leyden  jar  form  of  condenser  are 
shown  in  Fig.  6.  J is  the  jar  condenser,  A is 
one  terminal  of  the  coil  and  B the  other  terminal, 
G is  an  air  gap  and  should  be  just  at  the  limit 


20 


THE  RUHMKORFF  COIL. 


Fig.  6. 


HOW  TO  HANDLE  THE  COIL. 


21 


of  the  spark  length  so  that  the  sparks  will  just 
pass  across  it. 

The  jars  and  the  coil  must  be  insulated  from 
the  earth.  A few  porcelain  insulators  will  serve 
as  feet  for  the  apparatus,  or  a sheet  of  glass,  or 
in  an  emergency  even  perfectly  dry  and  clean 
dinner  plates  may  be  used. 

The  discharger  is  set  far  enough  apart  so  that 
there  is  no  discharge  across  H. 

The  Leyden  jar,  or  condenser,  shown  in  the 
figure,  is  made  out  of  a glass  jar,  a cork,  and  some 
tin-foil. 

The  jar  may  be  one  holding  from  one  pint  to 
one  quart.  The  larger  it  is,  the  longer  it  takes 
to  charge,  and  the  more  charge  it  holds. 

The  cork  is  varnished  with  shellac,  and  a cop- 
per or  iron  wire  is  thrust  down  through  its  centre. 

A brass  ball  of  one  inch  or  so  in  diameter  is 
fastened  to  the  upper  end  of  this  wire. 

Around  the  outside  of  the  jar  is  a sheet  of 
tin-foil  attached  thereto  by  shellac  varnish. 

This  must  lie  flat,  and  should  come  only  half 
way  up  the  jar,  as  shown  in  the  figure. 

A sheet  of  tin-foil  is  likewise  to  be  fastened  to 
the  inside  of  the  jar  parallel  with  the  outside  sheet. 

The  easiest  way  to  do  this  is  to  first  roll  up  the 
foil  into  a loose  roll.  Then  with  a brush,  shellac 
the  inside  of  the  jar  and  lay  it  flat  on  the  table. 
Insert  the  roll  of  foil  as  in  Fig.  7,  allow  it  to  ad- 
here to  the  shellac,  and  with  a stick  unroll  it  so 
that  it  conforms  to  the  shape  of  the  jar  J.  It 


22 


THE  RUHMKORFF  COIL. 


will  make  this  operation  easier  if  the  jar  be  turned 
at  the  same  time;  the  weight  of  the  foil  will  thus 
help  to  unroll  it.  With  a mop  made  of  a piece  of 
rag  on  a stick,  the  foil  T can  be  pressed  flat  on 
the  inside  surface  of  the  jar  and  smoothed  out, 
Fig.  8.  In  the  bottom  of  the  jar  thrust  a sheet 
or  two  of  foil  crumpled  up  so  as  to  touch  the 
inside  sheet  all  around  the  sides. 

Let  the  wire  from  the  cork  make  contact  with 


this  crumpled  foil  and  thus  connect  the  wire  to 
the  inside  sheet. 

Push  the  cork  home  and  the  Leyden  jar  is 
ready  for  use. 

Very  efficient  Leyden  jars  can  be  made  out  of 
thin  glass  tumblers.  The  only  object  of  the  cork 
being  to  exclude  dust  and  moisture  and  to  hold 
the  wire  it  may  be  dispensed  with.  Glass  plate 
condensers  are  easily  made.  They  are  merely 
sheets  of  glass  a foot  or  less  square  and  about  £ 


HOW  TO  HANDLE  THE  COIL. 


23 


of  an  inch  thick,  to  which  tin-foil  is  fastened  on 
both  sides,  a margin  being  left  around  the  edge 
to  prevent  discharges  taking  place  from  side  to 
side.  This  margin,  on  both  sides,  should  be 
equal  to  the  spark  length  between  the  foil,  that  is, 
the  margin  measured  from  the  foil  on  one  side 
to  the  foil  on  the  other  side  should  be  twice  the 
spark  length  used  to  charge  the  condenser. 

Condensers  must  always  be  insulated  from  the 
table  upon  which  they  stand.  This  can  be  done 
by  laying  them  upon  a sheet  of  glass  or  standing 
them  upright  in  a varnished  wood  holder. 


CHAPTER  IL 


Experiments  with  the  Spark. 

Most  of  the  experiments  with  sparks  should  be 
made  in  a darkened  room;  then  only  can  their 
true  beauty  be  realized. 

Increase  of  Spark  Length.  Attach  to  one  ter- 
minal of  the  coil,  Fig.  9,  a metal  disc  D,  and  to 
the  other  terminal,  a pointed  rod  or  wire  P. 


Fig.  9. 


Note  the  increased  length  of  the  spark  over  that 
obtained  with  two  pointed  wires. 

Substitute  a brass  ball  about  one  inch  in  diam- 
eter for  one  wire,  and  then  use  a ball  for  both  wires- 
Use  different  sized  balls  and  note  the  spark  char- 
acteristics. 

Separate  the  balls  so  that  no  spark  passes,  and 
watch  the  brush-discharge  in  the  air. 

24 


EXPERIMENTS  WITH  THE  SPARK. 


25 


It  will  be  seen  that  increasing  the  spark  gap 
and  adding  large  electrodes,  gives  a longer  spark, 
but  one  of  less  frequency.  The  energy  used  is 
the  same,  and  it  seems  as  if  the  spark  needed 
to  accumulate  power  to  jump  a long  gap. 

The  effect  is  similar  to  increasing  the  capacity 
of  a Leyden  jar  or  condenser.  It  takes  longer  but 
the  discharge  is  more  intense. 

A lighted  match  or  other  source  of  heat  will 
increase  the  sparking  distance  if  held  under  the 
spark  gap. 

Spark  Length  and  Voltage.  Various  estimates 
have  been  made  of  the  actual  voltage  necessary 
to  produce  a spark  of  a given  length. 

Up  to  quite  recently  there  has  been  a want  of 
uniformity  in  these  estimates,  but  the  researches 
of  Chas.  P.  Steinmetz  have  established  a set  of 
figures  which  are  as  accurate  as  can  be  obtained. 

In  general  the  voltage  necessary  to  produce  a 
spark  in  free  air  between  needle  points  is  as  follows: 

For  a gap  of  .25  inch,  4250  volts.  For  .5  inch 
gap,  10,000  volts.  For  a gap  one  inch  wide  the 
voltage  will  be  20,400  volts.  A two  inch  gap 
will  require  35,200  volts  and  a six  inch  gap  about 
70,000  volts. 

It  will  be  seen  that  the  voltage  does  not  in- 
crease in  direct  proportion  to  the  increase  in  the 
distance  between  the  points.  The  observed  volt- 
ages if  plotted  in  a curve  will  show  a gradual 
lessening  of  the  voltage  in  proportion  to  the 
lengthening  of  the  air  gap. 


26 


THE  RUHMKORFF  COIL. 


The  above  figures  were  obtained  by  allowing 
the  spark  to  jump  an  air  gap  between  two  needle 
points  in  free  air. 

Disruptive  Effect  of  Spark.  Bring  the  sparking 
terminals  of  the  coil  near  together,  about  one  half 
the  free  sparking  distance,  and  interpose  a stout 
card. 

The  sparks  will  pierce  the  card,  leaving  clean 
holes  similar  to  those  obtained  by  using  a red  hot 
needle. 

A thin  book  can  also  be  pierced  in  like  manner, 
the  number  of  leaves  pierced  depending  upon  the 
spark  rating  of  the  coil. 

If  a thin  sheet  of  paper  be  laid  on  a metal  or 
carbon  plate,  a metal  point  may  be  used  to  trace 
characters  or  figures.  The  perforated  sheet  will 
act  as  a stencil  if  laid  on  a clean  sheet  of  paper 
and  India  ink  or  stencil  ink  be  worked  over  its 
surface. 

A thin  sheet  of  glass  may  also  be  pierced  but 
the  spark  must  be  kept  from  wandering  over  its 
surface. 

To  do  this,  take  two  lumps  of  paraffin  wax. 
Stick  them  on  each  side  of  the  glass  opposite  to 
each  other.  Then  push  the  sparking  points  through 
the  paraffin  until  they  touch  the  glass,  one  on  each 
side. 

These  experiments  show  the  disruptive  char- 
acter of  the  electric  spark.  The  term  disruptive 
means  a tearing  or  forcing  apart. 


EXPERIMENTS  WITH  THE  SPARK. 


27 


An  insulator,  although  it  may  have  good  in- 
sulating qualities,  is  yet  capable^of  being  penetrated 
by  currents  of  high  disruptive  power. 

For  instance,  dry  air  is  the  best  possible  form 
of  insulator  for  currents  of  low  voltage.  Though 
two  bare  ends  of  a wire  were  held  a minute  frac- 
tion of  an  inch  apart  and  current  from  even  a 
large  battery  applied,  the  current  would  not  jump 
across  the  air  gap,  and  a galvanometer  in  series 
with  the  battery  and  wires  would  not  indicate  the 
passage  of  current.  But  if  a piece  of  rubber 
were  interposed  so  that  the  bare  wires  touched  it 
on  each  side,  a sensitive  galvanometer  would  show 
current  passing  through  the  rubber. 

On  the  other  hand  if  the  voltage  applied  were 
very  high  it  might  easily  jump  a considerable  air 
gap,  but  perhaps  not  be  able  to  tear  its  passage 
through  the  rubber. 

This  is  one  of  the  proofs  that  all  substances, 
even  those  classed  as  insulators,  conduct  elec- 
tricity to  some  extent.  Some  of  them,  however, 
limit  the  leakage  up  to  the  point  where  they  are 
torn  apart  or  disrupted.  This  is  termed  the 
breaking  down  point  of  the  insulation. 

Polarity  of  Terminals.  To  determine  the  po- 
larity at  the  spark  points  several  methods  may  be 
used. 

Attach  to  the  terminals  short  thin  iron  or  steel 
wires.  Needles  will  do. 

Let  the  spark  pass  a few  times,  and  note  which 


28 


THE  RUHMKORFF  COIL. 


point  is  hottest  or  becomes  most  consumed.  That 
one  is  the  negative. 

Take  two  glass  tubes  and  pass  a piece  of  wire 
about  four  inches  long  by  three  eighths  in  diam- 
eter through  each  of  them.  Leave  a half  inch  of 
the  wire  projecting  from  one  end  of  each  tube 
and  several  inches  from  the  other. 


Fill  the  tubes  with  molten  paraffin  wax,  which 
when  cool  will  seal  in  the  wires. 

Attach  the  long  wires  to  the  coil  terminals  and 
immerse  the  other  ends  of  the  tubes  about  two 
inches  under  water,  as  in  Fig.  10. 

Gas  will  be  given  off  from  both  wires ; hydrogen 
at  one,  oxygen  at  the  other.  The  amount  of 


EXPERIMENTS  WITH  THE  SPARK. 


29 


hydrogen  at  the  negative  pole  will  be  twice  that 
of  the  oxygen  at  the  positive  pole. 

The  difference  in  amount  is  readily  perceptible. 

The  spark  observed  in  a Geissler  or  vacuum  tube 
will  show  a purple  red  glow  at  the  positive  pole, 
and  a bluish  violet  at  the  negative  pole. 

Soak  blotting  paper  in  hot  solution  of  starch 
and  let  it  dry.  Then  dip  the  paper  into  a solu- 
tion composed  of  iodide  of  potash  one  half  ounce, 
water  one  pint. 

This  testing  paper  is  to  be  used  wet,  though  it 
may  be  kept  dry  and  moistened  when  about  to  be 
used. 

Let  sparks  pass  between  the  coil  terminals  held 
apart  but  near  enough  to  the  paper  so  that  the 
sparks  shall  strike  it. 

The  paper  near  the  positive  pole  will  turn  to  a 
violet  brown. 

Production  of  Ozone.  Ozone  which  is  a form  of 
oxygen  is  given  off  at  the  secondary  spark  gap  and 
is  soon  noticeable  through  its  peculiar  smell  as  of 
phosphorous. 

It  is  not  dangerous  if  inhaled  in  small  quantities 
mixed  with  air,  but  is  likely  to  cause  a headache. 

Ozone  is  used  in  various  chemical  applications, 
for  bleaching  and  for  purifying  the  air. 

If  two  sheets  of  tin-foil  be  loosely  laid  on  each 
other  but  separated  by  air  gaps,  the  sparks  pro- 
duced will  liberate  ozone  quite  freely.  It  will 
also  be  quite  freely  produced  if  the  two  sheets 


30 


THE  RUHMKORFF  COIL. 


are  suspended  parallel  to  each  other  but  separated 
so  that  only  an  occasional  spark  passes. 

It  is  not  a particularly  pleasing  experiment, 
however,  and  only  serves  to  show  one  of  the  chem- 
ical effects  of  the  high  tension  discharge. 

Scintillating  Sparks.  Lay  a plate  of  glass  be- 
tween the  terminals  of  the  coil  and  bend  down 
the  discharger  so  that  its  ends  lie  on  the  plate. 
If  the  dust  from  a scraped  lead  of  a pencil  be 


Fig.  11. 


now  strewn  over  the  plate,  the  spark  will  travel 
between  a longer  air  gap  than  in  the  air. 

The  carbon  dust  particles  will  glow  like  miniature 
stars. 

Substitute  iron  filings  for  the  powdered  carbon, 
and  the  color  of  the  stars  will  be  different. 

Clean  the  glass  and  breathe  upon  it.  The 
sparks  will  travel  over  a greater  distance  and  will 
branch  out  as  in  Fig.  11.  They  will  be  violet  in 
color  and  emit  ozone  freely. 

Take  a piece  of  glass  tube  several  times  as  long 


EXPERIMENTS  WITH  THE  SPARK. 


31 


as  the  free  spark  in  the  air  and  shake  some  carbon 
dust  as  that  from  a lead  pencil  into  it. 

The  spark  will  wander  through  the  tube  and 
render  it  luminous  with  innumerable  sparkling 
points. 

If  the  terminals  be  laid  on  opposite  edges  of  a 
gilded  book-cover,  the  sparks  will  be  greatly  in- 
creased in  length  and  will  run  all  over  the  gilding. 
This,  however,  is  soon  destructive  to  the  gilding, 
and  if  the  latter  be  not  of  gold  leaf,  will  turn  it 
black. 

Coat  a plate  of  glass  with  tin  foil,  or  metallic 
paint  such  as  gold  paint. 

When  it  is  thoroughly  dry,  scratch  lines  across 
the  paint  or  foil  crossing  each  other.  The  lines 
should  cut  clear  down  to  the  glass. 

Upon  connecting  opposite  sides  of  the  foil  or 
paint  covered  surface  to  the  coil,  minute  sparks 
will  travel  across  it. 

If  one  terminal  be  held  just  over  the  centre  of 
the  plate,  the  sparks  will  jump  down  and  rush 
across  the  metallic  particles  in  fiery  streams. 

If  the  coil  be  powerful  enough  to  cause  the 
whole  plate  to  spark,  designs  may  be  cut  out  of 
card  and  laid  on  the  plate. 

They  will  then  stand  out  dark  against  a starry 
background. 

Condenser  Discharges.  Connect  up  the  con- 
denser as  for  charging,  Fig.  6,  but  bring  the  dis- 
charger points  within  sparking  distance. 


32 


THE  RUHMKORFF  COIL. 


The  sparks  will  be  much  increased  owing  to  the 
condenser  adding  its  discharge  to  that  of  the  coil. 

On  each  side  of  a pane  of  glass  one  eighth  of  an 
inch  thick  by  one  foot  square  stick  a sheet  of  tin- 
foil  ten  inches  square.  This  will  leave  a margin 
of  one  inch  all  around  the  plate. 

Lay  the  plate,  which  is  now  a condenser,  on  top 
of  a tumbler  and  run  wires  from  the  terminals 
of  the  coil  to  opposite  sides  of  the  condenser. 
One  wire  is  to  touch  the  foil  on  one  side,  and  the 
other  is  to  be  just  within  sparking  distance  from 
the  other  foil  sheet. 

Allow  a few  sparks  to  pass  and  the  condenser 
will  be  charged. 

Bring  a piece  of  bent  wire,  held  by  an  insulating 
handle,  to  the  condenser  so  that  each  point  comes 
near  the  opposite  foil  surfaces.  The  condenser  will 
discharge  with  a sharp  crack  and  a brilliant  spark. 

Bring  a wire  out  from  each  foil  sheet  and  sep- 
arate the  ends  by  a short  air  gap.  Connect  by 
two  more  wires  the  foil  sheets  to  the  coil. 

Upon  setting  the  coil  in  operation,  sparks  will 
pass  across  the  air  gap  with  a loud  crackling  noise. 

With  a large  condenser  and  a powerful  coil  the 
noise  is  almost  unbearable. 

Spark  Over  Oil.  Immerse  in  a tumbler  of  water 
one  of  the  glass  tubes  before  referred  to  into 
which  a thin  wire  there  has  been  sealed  with  par- 
affin. Pour  on  the  surface  of  the  water  a tea- 
spoonful of  oil  or  turpentine. 


EXPERIMENTS  WITH  THE  SPARK. 


33 


Connect  the  wire  leading  into  the  tube  to  one 
terminal  of  the  coil,  and  the  other  terminal  to  a 
wire  held  over  the  surface  of  the  oil. 

Greenish-white  crackling  sparks  will  pass  from 
the  free  wire  to  the  oil  and  a bluish  flame  play 
near  its  surface. 

Illuminated  Objects.  Into  a lemon  or  orange, 
push  two  stout  wires  connected  to  the  terminals 
of  the  coil.  They  should  be  inserted  from  op- 
posite sides  so  as  to  leave  an  air  gap  near  the 
centre  of  the  fruit. 

Upon  passing  sparks  through  the  wires  the  fruit 
will  light  up  in  a most  striking  manner.  A cake 
of  toilet  soap  will  also  light  up. 

If  one  terminal  be  inserted  in  the  soap  and  the 
other  be  held  within  sparking  distance  from  it, 
yellow  flames  will  result.  The  best  soap  for  this 
purpose  is  the  cheaper  kind  containing  a larger 
percentage  of  free  alkali. 

Luminous  Designs.  Coat  a sheet  of  glass  on 
both  sides  as  in  making  a condenser,  but  let  the 
margin  be  less  than  one  half  the  spark  length  of 
the  coil. 

Draw  a design  on  the  foil  on  one  side,  as  an 
initial  letter,  a monogram,  or  a star,  then  bring 
all  good  designs  for  the  purpose. 

Remove  the  foil  from  around  the  design  so  that 
the  design  stands  out  clear. 

Connect  the  foil  on  the  back  of  the  plate  to  one 


34  THE  RUHMKORFF  COIL. 

terminal,  and  that  on  the  other  side  to  the  other 
terminal. 

In  the  dark  the  design  will  stand  out  brightly, 
a pale  luminescence  streaming  from  its  edges. 
The  glass  must  be  thin  or  the  experiment  may 
fail. 

Such  a design  used  in  a similar  manner  with 
the  Tesla  coil  described  later,  will  show  up  most 
brilliantly. 


Electric  Arc.  Point  two  pieces  of  black  lead 
pencil,  then  bare  a portion  of  the  black  lead  at 
the  unpointed  end  of  each,  and  twist  on  pieces  of 
fine  copper  wire. 

Connect  these  two  wires  to  the  coil  terminals, 
and  support  the  pencils  on  top  of  two  glass  tum- 
blers or  other  insulating  stands. 

Bring  the  pointed  ends  together  but  leaving 


EXPERIMENTS  WITH  THE  SPARK. 


35 


an  air  gap  of  about  one  quarter  the  normal  spark 
length  of  the  coil  between  them  as  in  Fig.  12. 

With  a little  adjustment  to  and  from  each 
other,  the  pencil  points  will  form  an  electric  arc, 
as  in  an  arc  lamp,  and  give  an  intense  white  light. 

Vibrator  of  coil  should  work  very  rapidly  to 
get  best  results. 

Cohesion  Figures.  The  Strethill  Wright  cohesion 
figures  can  be  produced  with  various  solutions. 

Take  a clean  glass  plate  or  preferably  a thin 
sheet  of  mica.  Lay  it  on  a metal  plate  or  piece 
of  tin-foil  connected  to  one  coil-terminal. 

Place  a drop  of  a saturated  solution  of  potassium 
cyanide  in  the  centre  and  in  the  drop  put  the  point 
of  a wire  connected  to  the  other  terminal.  Care 
must  be  taken  in  this  experiment,  as  Potassium 
cyanide  is  a most  powerful  poison. 

When  the  coil  is  started,  branches  will  radiate 
from  the  centre  of  the  drop  in  beautiful  forms.  A 
small  spark  only  is  necessary  therefore  at  first  reduce 
the  battery  power. 

Coin  Images  from  Spark.  Lay  a clean  sheet  of 
glass  such  as  a photographic  plate  on  a sheet  of 
tin-foil,  and  lay  a coin  in  the  centre  of  the  glass 
plate. 

Connect  the  tin-foil  to  one  terminal  of  the  coil 
and  the  coin  by  a wire  to  the  other  terminal. 
Allow  several  sparks  to  pass. 

After  a few  seconds,  during  which  the  sparks 


36 


THE  RUHMKORFF  COIL. 


have  been  passing,  remove  the  coin  carefully  and 
breathe  on  the  glass  where  the  coin  has  been  lying. 
An  image  of  the  coin  will  be  seen  distinctly  on 
the  glass  plate. 

If  the  tin  foil  has  not  been  first  attached  to  the 
plate  by  means  of  some  form  of  cement  it  will  be 
found  to  adhere  after  the  current  has  ceased  and  the 
plate  is  raised. 

The  image  will  reappear  several  times  after  the 
breath  has  dried  off  the  plate,  and  if  the  plate  is 
breathed  upon  again,  it  will  often  stand  being  wiped 
with  a cloth  and  yet  reappear. 

This  does  not  seem  to  be  due  so  much  to  a 
change  in  the  glass,  as  to  particles  of  the  coin 
or  perhaps  dust  adhering  to  it. 

Spark  Photographs.  Lay  a photographic  dry 
plate,  emulsion  side  up,  on  a sheet  of  metal  or 
tin-foil  in  an  absolutely  dark  room.  Connect 
the  metal  plate  to  one  coil  terminal  and  bring 
the  point  of  the  other  terminal  to  the  centre  of  the 
emulsion.  Separate  the  discharger  points  about 
half  the  sparking  distance  of  the  coil. 

Cause  a very  few  sparks  to  pass,  then  remove 
and  develop  the  plate. 

Markings  will  be  found  resembling  tree  branches, 
or  tangled  thread  the  characteristics  being  de- 
pendent upon  which  terminal  was  near  the  emulsion. 

Some  of  the  designs  to  be  made  this  way  are 
extremely  beautiful  and  will  repay  the  cost  of 
the  plate  and  chemicals. 


EXPERIMENTS  WITH  THE  SPARK. 


37 


There  are  several  causes  which  change  the 
figures  and  never  will  the  same  figure  be  pro- 
duced twice. 

At  the  same  time  certain  characteristics  will 
reappear,  depending  upon  the  polarity  of  the  sparks. 

Revolving  Disc.  Cut  out  a disc  of  pasteboard 
a foot,  or  less,  in  diameter,  and  mark  on  it  in 


jet  black  ink  some  such  figure  as  in  Fig.  13. 

Stick  a pin  or  thumb  tack  through  the  center 
and  mount  it  on  a strip  of  wood,  or  so  that  it 
may  be  revolved  rapidly. 

In  a dark  room  hold  the  disc  as  near  to  the 
discharger  as  possible,  start  the  coil  vibrator  and 
let  the  light  from  the  sparks  illuminate  the  disc 
while  it  is  spinning  very  curious  figures  will  be 


Fig.  13. 


38 


THE  RUHMKORFF  COIL. 


seen,  according  to  the  relation  in  speed  between 
the  spinning  disc  and  the  sparks  per  second. 

One  of  the  lessons  taught  by  this  experiment 
is  that  the  coil  spark  is  not  continuous  but  inter- 
mittent. 

A scientific  modification  of  this  apparatus, 
using  revolving  mirrors,  is  employed  to  show  that 
the  discharge  of  a condenser  or  Leyden  jar  is 
intermittent. 

Powder  Figures.  Lay  a sheet  of  glass  on  a 
metal  plate  connected  with  one  terminal  of  the 
coil.  Dust  on  the  glass  enough  lycopodium 
powder  to  just  cover  the  surface. 

This  powder  can  be  obtained  from  almost  any 
druggist. 

Approach  a wire  from  the  second  terminal  of 
the  coil  to  the  centre  of  the  glass  surface  and 
curious  figures  will  be  obtained. 


CHAPTER  III. 


Effects  in  the  Vacuum. 

Fluorescence.  There  are  several  substances  and 
fluids  which  possess  fluorescent  qualities.  The 
word  is  hard  to  define,  but  refers  to  the  peculiar 
colors  which  are  displayed  by  them  under  the 
light  of  the  electric  spark. 

In  some  cases  the  substance  experimented  upon 
will  glow  for  a long  time  after  the  spark  has  ceased. 

Although  the  colors  may  be  seen  with  the  aid 
of  the  spark  alone,  the  light  from  a Geissler  tube 
is  preferable  as  the  illumination  is  more  general. 

Dissolve  bisulphate  of  quinine  in  acidulated 
water.  This  can  best  be  done  by  a druggist.  In 
general  60  grains  of  quinine  to  the  ounce  of  water 
is  sufficient. 

Write  with  this  solution  on  a piece  of  white 
paper.  When  the  writing  is  held  under  the  light 
of  the  spark,  or  that  of  a Geissler  tube,  the  letters 
will  stand  out  in  a pale  blue. 

The  solution  may  also  be  placed  near  the  light 
of  the  spark  while  contained  in  a porcelain  dish. 
But  it  will  not  often  fluoresce  while  in  the  bottle 
as  the  ordinary  glass  seems  to  destroy  the  fluores- 
cent qualities. 


40 


THE  RUHMKORFF  COIL. 


A block  of  Willemite  will  show  a vivid  green 
tint  under  the  same  circumstances.  Willemite 
is  one  of  the  forms  of  zinc  ore,  and  can  be  bought 
for  about  forty  cents  a pound  from  large  whole- 
sale drug  dealers  or  chemical  supply  houses. 

Glass  containing  uranium  glows  with  an  apple 
green  color  under  the  light  of  the  spark. 

Vacuum  Test.  Hold  a bumed-out  incandescent 
lamp  by  the  globe  end  and  bring  the  base  end 


near  one  of  the  coil  terminals  as  in  Fig.  14.  If 
the  lamp  has  a poor  vacuum,  as  is  likely  in  an  old 
lamp,  a bluish  glow  will  fill  the  bulb. 

In  testing  new  incandescent  lamps  for  vacuum 
this  glow  indicates  low  vacuum,  and  lamps  show- 
ing this  glow  are  rejected. 

Either  end  of  the  lamp  may  be  brought  to  the 
coil  terminal.  Sometimes  the  glow  is  obtained 
from  one  terminal  and  not  the  other. 


EFFECTS  IN  THE  VACUUM. 


41 


Often  when  the  carbon  filament  is  not  broken, 
the  latter  will  vibrate  and  give  out  a bell  like  sound. 

Lamps  giving  this  glow  were  used  as  X-ray 
tubes  in  the  early  days  of  X-ray  experiments. 

Geissler  Tubes.  For  those  who  do  not  possess 
an  air  pump  of  any  sort,  the  beautiful  effects  of 
the  spark  in  rarefied  air  or  gases  can  be  obtained 
by  means  of  Geissler  tubes. 

For  the  most  part  Geissler  or  vacuum  tubes 
are  made  abroad,  those  made  in  the  United  States 
although  more  efficient  are  also  more  expensive. 

As  it  is  possible  to  almost  instantaneously  ruin 
an  expensive  tube  it  is  best  to  commence  opera- 
tions with  the  cheaper  ones. 

Although  no  actual  figures  can  be  given  for  the 
reason  that  the  degree  of  vacuum  varies,  a six 
inch  tube  requires  about  one  eighth  inch  spark. 

It  is  unwise  to  place  a small  tube  alone  in  series 
with  a large  spark  coil. 

A number  of  tubes  should  be  arranged  in  series 
allowing  not  less  than  two  feet  of  tube  to  each 
inch  of  spark,  or  a spark  gap  may  be  interposed 
between  the  tube  and  the  coil. 

Although  brighter  colors  are  obtained  by  forcing 
a tube  above  this  maximum,  it  is  at  the  expense 
of  the  tube,  particularly  if  it  be  of  the  cheaper 
foreign  make. 

Geissler  tubes  containing  a small  percentage  of 
rarefied  gas  will  show  colors  according  to  the  gas 
used. 


42 


THE  RUHMKORFF  COIL. 


Hydrogen  gives  blue  and  red  tints;  nitrogen 
pink;  common  coal  gas  green;  and  carbon  dioxide 
white.  Carbon  dioxide  is  the  gas  given  oft  by 
the  burning  of  coke  or  charcoal,  and  is  one  of  the 
waste  products  of  combustion  in  a furnace. 

The  light  from  carbon  dioxide  is  the  nearest  to 
white  light  without  heat  that  has  yet  been  pro- 
duced. But  it  has  not  a strong  illuminating  power. 


Mercury  vapor  gives  a strong  light  but  has  a 
peculiar  green  tint,  as  may  be  seen  in  the  Cooper- 
Hewitt  lamp. 

If  the  experimenter  has  a means  of  exhausting 
a tube,  the  variety  of  color  effects  which  may  be 
produced  is  very  great. 

A tube  is  first  filled  with  a gas  and  then  con- 
nected to  an  air  pump.  As  the  tube  is  exhausted, 
the  color  effects  become  visible  providing  the  spark 


EFFECTS  IN  THE  VACUUM. 


43 


is  passing  at  the  time,— and  these  change  with 
the  amount  of  exhaustion. 

In  Fig.  15  are  shown  two  of  the  simple  forms  of 
Geissler  tubes.  The  top  one  A has  bulbs  of 
Uranium  glass  blown  in  it,  which  glow  with  an 
intense  apple  green  color. 

The  lower  tube  B has  an  outer  envelope  of 
glass,  provided  with  a short  tube  and  cork  through 
which  different  solutions  may  be  poured. 

A solution  of  quinine  when  used  gives  a pale 
blue  tint.  The  spark  in  the  inner  tube  shows 
through  this. 

Hold  a Geissler  tube  by  one  end  and  approach 
the  other  end  to  the  coil  terminal.  The  tube  will 
light  up  faintly. 

Lay  the  tube  parallel  with  the  coil  and  a few 
inches  from  it,  a faint  glow  will  be  seen.  Connect 
the  tube  to  the  coil  by  pieces  of  wire,  the  finest 
wire  will  suffice  even  No.  36  B.  & S.  gauge. 

The  tube  must  not  touch  the  coil  itself  but  should 
be  suspended  in  a stand  or  laid  across  a block  of 
dry  wood. 

While  the  tube  is  glowing  with  the  passage  of 
the  spark  bring  a finger  near  the  tube  and  a 
bright  spot  will  follow  it.  The  luminosity  is 
deflected  by  the  finger. 

A permanent  magnet  will  also  exhibit  similar 
power. 

Geissler  Tube  Rotator.  The  effects  to  be  ob- 
tained from  an  active  Geissler  tube  while  it  is 


44 


THE  RUHMKORFF  COIL. 


rotating,  far  surpass  those  from  a tube  lying 
stationary. 

By  varying  the  rate  of  vibration  in  the  contact 
breaker,  and  the  speed  at  which  the  tube  rotates, 
many  striking  and  curious  phenomena  are  pro- 
duced. 

As  the  spark  is  but  momentary,  at  times  the 
tube  will  seem  to  stand  still.  Then  as  the  rotation 
increases  in  speed  there  will  appear  to  be  two 
tubes.  Then  more,  until  at  a certain  speed  there 
will  seem  to  be  a vast  number,  all  radiating  from 
a common  centre. 

The  tube  may  be  mounted  on  a pasteboard  wheel 
which  in  turn  is  fixed  to  a shaft. 

A simple  way  is  to  glue  on  either  side  of  the  wheel 
centre,  an  ordinary  cotton  spool. 

Through  both  spools  and  the  wheel  passes  a 
shaft  which  can  be  held  in  bearings  made  of 
strips  of  wood,  and  fastened  to  a wooden  base. 
Around  each  spool  a piece  of  tin-foil  is  wrapped 
and  glued. 

The  tube  being  tied  with  thread  across  the 
wheel  is  connected  by  pieces  of  fine  wire,  one 
terminal  going  to  the  foil  on  each  spool. 

Two  pieces  of  wire  are  held  by  screws  to  the 
base  so  that  each  piece  just  clears  the  foil  on  each 
spool. 

By  connecting  these  wire  brushes  to  the  coil 
terminals,  the  tube  will  receive  current  and  light  up. 

The  wheel  can  then  be  rotated  by  hand  or  by 
an  electric  motor. 


EFFECTS  IN  THE  VACUUM. 


45 


In  Fig.  16  S S are  the  spools,  B B the  brushes 
connecting  at  C C to  the  coil,  and  U is  the  upright 


into  which  the  shaft  is  fixed.  The  spools  turn 
freely  on  this  shaft  but  are  kept  from  coming  off 
by  the  pin  P.  The  wheel  W should  be  stout 


46 


THE  RUHMKORFF  COIL. 


enough  not  to  bend  when  being  turned.  The  spark 
will  jump  from  B B to  the  foil  bands  around 
the  spools,  so  no  actual  contact  or  friction  is 
necessary. 

By  varying  the  angle  at  which  the  tube  lies 
on  the  face  of  the  wheel  the  number  of  effects  is 
vastly  increased. 

Production  of  a Vacuum.  Many  of  the  most 
beautiful  experiments  in  high-tension  electricity 
require  the  production  of  a partial  vacuum. 

The  air  has  to  be  exhausted  from  a glass  tube 
or  bulb  but  it  is  not  necessary  that  the  highest 
degree  of  exhaustion  be  obtained.  In  fact,  as 
will  be  seen  later,  there  is  a limit  in  this  exhaustion 
beyond  which  it  is  useless  to  go. 

The  action  of  a syringe  or  piston  pump  is  too 
well  known  to  require  description  here,  but  it 
may  be  noted  that  the  better  the  piston  fits  the 
cylinder,  the  better  the  pump  works,  the  leakage 
being  so  much  less. 

Mercury  flowing  through  a glass  tube  acts  as  a 
piston,  but  the  liquid  nature  of  mercury  permits 
it  to  follow  the  surface  of  the  tube  far  more  closely 
than  will  a solid  metal  piston,  and  its  weight 
permits  it  to  act  by  gravitation. 

A vacuum  in  a glass  tube  can  be  easily  produced 
by  filling  the  tube  with  mercury  and  allowing  it 
to  run  almost  out.  Of  course  the  tube  must  be 
first  closed  at  the  other  end. 

In  the  space  originally  occupied  by  the  mercury, 


EFFECTS  IN  THE  VACUUM. 


47 


the  air  will  be  found  to  be  highly  rarefied  and  a 
partial  or  low  vacuum  found  therein. 

Mercury  pumps  of  this  order  will  be  described 
later. 

Owing  to  the  fragility  of  the  glass  tubes  and 
the  undesirability  of  handling  mercury,  piston 
pumps  are  preferable  as  the  vacuum  needed  is  not 
high. 

There  are  several  simple  piston  air  pumps  on 
the  market  a form  consisting  merely  of  a cylinder, 
valves,  and  piston  costing  but  a few  dollars. 

There  is  no  great  difficulty  about  making  an  air 
exhausting  pump  out  of  a common  bicycle  air 
compressing  pump,  only  the  valve  action  will  have 
to  be  changed. 

Vacuum  Obtained  with  Mercury.  Procure  a piece 
of  glass  tubing  three  feet  or  so  in  length  and  about 
three  eighths  inch  internal  diameter.  Holding  it 
by  an  end  in  either  hand,  bring  the  middle  near 
a gas  flame ; gradually  approaching  until  the 
centre  portion  of  the  tube  is  red  hot. 

The  object  in  not  immediately  thrusting  the 
tube  in  the  flame,  is  that  it  may  first  get  hot  and 
thus  not  crack. 

When  the  glass  is  red  hot  or  soft  enough  to 
bend  gently  form  it  into  the  shape  of  an  inverted  U. 

Remove  the  tube  slowly  from  the  flame  and  set 
it  aside  to  cool.  Do  not  lay  it  down  immediately 
on  a cold  surface  or  it  will  crack. 

When  ready  to  use,  set  two  small  tumblers  on 


48 


THE  RUHMKORFF  COIL. 


the  table  so  that  one  leg  of  the  tube  will  go  in 
each,  Fig.  17. 

Fill  the  tube  with  mercury,  then  invert  it  and 
place  a leg  of  the  tube  in  each  tumbler.  As  the 
mercury  runs  out  into  the  tumblers  it  will  leave 
a vacuum  in  the  tube. 


This  simple  device  can  be  used  to  show  most 
of  the  experiments  due  to  the  spark  in  rarefied 
air  or  gas.  The  vacuum  must  not  be  as  perfect 
as  can  possibly  be  obtained,  as  in  that  event,  the 
spark  will  not  pass  at  all.  An  electric  spark 
passes  with  difficulty  in  air  or  gas  under  compres- 
sion, or  in  air  or  gas  in  a high  state  of  rarefication. 


EFFECTS  IN  THE  VACUUM. 


49 


The  average  degree  of  vacuum  will  soon  be  found 
by  experiment. 

Into  each  tumbler  place  a wire  leading  from  the 
coil  terminals  so  that  the  tube  is  in  series  with 
the  spark  gap.  Upon  starting  the  vibrator,  the 
tube  will  glow  with  most  beautiful  colors. 

Mercury  Pumps.  The  simplest  form  of  mercury 
pump  is  the  Sprengel, 

The  Sprengel  pump  (Fig.  18)  consists  of  a glass 
tube  F about  40  inches  or  more  in  length  and  one 
eighth  of  an  inch  internal  diameter.  The  tube 
must  be  of  considerable  thickness,  at  least  one 
eighth  of  an  inch,  or  it  will  soon  crack. 

At  the  top  of  the  tube  is  a funnel  or  reservoir  R 
into  which  mercury  can  be  poured.  The  size  of 
R is  easiest  determined  by  the  amount  of  mercury 
available.  A cock  5 regulates  the  mercury  which 
flows  from  R down  F and  into  an  open  vessel  V . 

A tube  E , also  provided  with  a cock,  is  melted 
on  to  F near  the  top.  The  bulb  or  vessel  to  be 
exhausted  of  air  is  attached  at  E , the  cock  5 
opened,  and  mercury  allowed  to  drop  down  the 
tube  F into  V. 

As  each  drop  rushes  down  F and  passes  the  hole 
leading  into  E,  it  sucks  the  air  out  of  E , acting 
like  a piston.  Each  succeeding  drop  draws  air 
after  it  until  the  vessel  attached  at  E is  exhausted. 

When  first  set  in  operation,  the  drops  fall 


50 


THE  RUHMKORFF  COIL. 


Fig.  18- 


EFFECTS  IN  THE  VACUUM. 


51 


silently  because  of  the  air  cushions  between  them. 
After  a while  this  air  becomes  scarce  and  the 
drops  fall  with  a sharp  click,  giving  an  indication 
that  the  exhausting  is  in  progress. 

As  the  atmospheric  pressure  holds  it  up,  the 
mercury  in  F will  remain  at  about  H , that  is,  30 
inches  from  its  level  in  V. 

The  Sprengel  pump  is  not  perfect,  and  a higher 
degree  of  exhaustion  can  be  obtained  with  other 
pumps,  but  there  is  none  easier  to  construct  or 
handle,  and  its  power  is  more  than  ample  for  all 
induction  coil  work. 

A receiver  may  be  connected  to  E,  with  a valve 
so  that  E may  be  disconnected.  In  laboratory 
forms,  the  receiver  is  generally  a heavy  bell  shaped 
glass  globe.  It  is  turned  down  over  the  orifice 
of  the  tube  attached  to  E , being  sealed  by  a rubber 
or  similar  ring  around  the  base,  to  a plate  or  stand. 

In  coil  construction  it  is  an  advantage  to  ex- 
haust the  air  from  between  the  layers  of  wire, 
that  the  insulating  material  used  may  tak , its  place. 

Placing  the  coil  in  an  insulating  bath  under  the 
receiver  of  an  air  pump,  and  exhausting  the  air, 
will  cause  the  air  in  the  coil  to  bubble  up  to  the 
surface  of  the  hot  insulation. 

In  the  case  of  a home-made  pump  of  this  de- 
scription, the  tube  F may  be  joined  to  R by  a 
piece  of  flexible  rubber  tubing. 

A pinch  cock,  similar  in  principle  to  the  spring 
clip  used  to  hold  papers,  will  pinch  the  rubber 
tube  and  shut  off  the  flow  of  mercury. 


52 


THE  RUHMKORFP  COIL. 


A glass  funnel  for  R may  be  bought  from  any 
druggist  or  photographic  supply  store. 

The  tube  is  held  to  a board  framework  by 
wood  cleats  as  shown,  a piece  of  cloth  under 
the  cleat  will  lessen  the  danger  of  breaking  the 
tube. 

The  outlet  tube  E can  be  blown  on  by  any 
glass  blower,  or  a short  joint  of  this  sort  procured 
through  a philosophical  instrument  dealer.  If  a 
short  joint  is  obtained,  it  may  be  attached  between 
the  top  piece  and  the  bottom  piece  by  rubber 
tube  and  clamped  rigidly. 

The  end  of  E should  be  turned  up,  or  cemented 
into  an  elbow  having  a screw  thread  cut  on  its 
vertical  arm.  On  this  thread  may  be  screwed 
glass  vessels  provided  with  brass  unions. 

Mechanical  or  piston  pumps  will  not  be  described 
here,  as  they  more  rightly  belong  to  books  on 
mechanical  construction. 

It  may  be  desired  to  attach  objects  to  the 
exhausting  pipe  without  metal  screw  unions,  and 
in  that  case  a cement  may  be  used. 

One  good  receipe  is  paraffin  wax  three  parts, 
vaseline  one  part,  melted  together  in  a hot  water 
bath.  The  pot  containing  the  wax  and  vaseline 
should  stand  in  a pan  of  boiling  water  until  the 
mixture  is  complete. 

Another  cement  is  Burgundy  pitch  96  parts, 
gutta  percha  four  parts.  Heat  and  melt  together. 
Beeswax  and  resin  in  equal  parts  also  makes  a 
good  cement. 


CHAPTER  IV. 

Wireless  Telegraphy  and  Tesla  Coils. 

Hertz  Experiments.  The  induction  experiments 
initiated  by  Professor  Hertz,  which  embody  the 
essential  principles  underlying  wireless  telegraphy, 
can  be  studied  with  a small  coil. 

Construct  a ring  of  stout  copper  wire  R two  or 
three  feet  in  diameter,  as  in  Fig.  19.  It  may  have 
the  two  brass  balls  on  its  ends  CD,  or  simply 
have  the  ends  pointed  and  separated  by  an  air 
gap  of  a fraction  of  an  inch. 

Set  the  ring  in  a vertical  position  or  hang  it 
up  by  a thread  from  the  top. 

Attach  to  each  rod  of  the  discharger,  a plate 
of  metal  eight  inches  square.  These  plates  must 
be  insulated  from  contact  with  any  other  object, 
but  the  discharger.  They  are  to  set  out  in  line 
with  the  rods. 

Thin  metal  balls  three  inches  in  diameter  will 
also  serve  the  same  purpose. 

Place  the  discharger  balls  A B just  far  enough 
apart  so  that  the  sparks  can  pass  freely,  and  set 
the  coil  in  operation. 

The  ring  must  hang  parallel  with  the  discharger 
rods  or  rather  with  the  spark  gap  of  the  coil. 

53 


54 


THE  RUHMKORFF  COIL, 


Fig.  19. 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  55 

At  first  let  the  distance  between  the  coil  and 
the  ring  be  six  feet. 

Upon  adjusting  the  spark  gap  of  the  ring  at 
C D , minute  sparks  will  be  seen  to  pass  across  it. 
The  ring  must  not  be  touched  while  the  sparks 
are  being  looked  for. 

Move  the  ring  gradually  farther  away  and  the 
sparks  will  pass  until  a position  is  reached  de- 
pending upon  the  strength  of  the  coil,  when  the 
sparks  will  not  pass. 

Make  two  flat  spirals  a few  feet  in  diameter 
of  copper  wire  similar  to  Fig.  20.  No.  10  B.  & S. 
hard  drawn  bare  copper  wire  is  easiest  to  handle 
in  the  form  of  a spiral.  In  order  to  preserve  the 
shape  of  the  spiral  lay  it  flat  on  a table  and  tie 
the  turns  together  with  thread,  as  shown  by  the 
dotted  lines.  When  it  is  lifted  up  the  thread  will 
keep  the  turns  at  the  right  distance  apart. 

The  turns  may  also  be  clamped  between  pieces 
of  wood  or  made  up  on  a board,  according  to  the 
desire  of  the  constructor.  But  no  metallic  threads 
or  wire  should  be  used  to  tie  with  or  the  turns 
will  be  short  circuited. 

Hang  the  two  spirals  vertically  and  parallel  with 
each  other.  They  should  be  several  feet  apart  at 
first  until  the  power  of  the  apparatus  can  be 
gauged. 

Connect  one  end  of  one  spiral  to  one  terminal 
of  the  coil,  and  hold  the  other  end  at  such  a dis- 
tance from  the  other  terminal  that  sparks  pass 
freely  into  the  spiral. 


56 


THE  RUHMKORFF  COIL. 


Sparks  will  pass  across  C A on  the  second 
spiral  as  in  the  former  experiment,  but  will  be 
much  enlarged  or  of  greater  frequency. 

If  a telephone  receiver  be  connected  to  C A, 


and  the  coil  contact-breaker  be  snapped,  there 
will  be  a click  in  the  receiver  at  each  spark. 

The  Morse  telegraph  code  can  be  thus  used  to 
signal  with. 

The  spirals  can  be  moved  farther  apart  as  in 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  57 


the  case  of  the  simple  rings  and  if  the  coil  be  power- 
ful enough,  may  be  in  a separate  room  from  the 
coil-connected  spiral. 

Similar  spirals  were  used  by  W.  H.  Preece  of  the 
British  Post-Office  Telegraph  System  many  years 
ago,  in  conducting  experiments  on  induction  at  a 
distance. 

Signals  were  sent  for  distances  of  several  thou- 
sand yards  and  across  a wide  river. 

Wireless  Telegraphy  with  Coherer.  In  wireless 
telegraphy,  advantage  is  taken  of  the  fact  that  an 
electric  wave  of  high  tension  lowers  the  resistance 
of  certain  devices  when  they  are  in  circuit  with 
them. 

The  coherer  is  a device  of  normally  high  re- 
sistance. It  is  in  series  with  a battery  and  per- 
haps a telegraph  relay. 

Normally  it  is  of  such  high  resistance  that  the 
battery  will  not  operate  the  relay  through  it. 

But  when  penetrated  by  the  wave  arising  from 
the  discharge  of  an  induction  coil,  its  resistance 
is  lowered  from  perhaps  a thousand  ohms  to  a 
fraction  of  an  ohm.  When  this  occurs  the  battery 
operates  the  relay. 

A coherer  can  be  made  from  a piece  of  glass 
tube  three  eighths  of  an  inch  in  diameter,  by  three 
inches  long. 

Metal  plugs  are  fitted  in  it  so  that  they  leave 
a space  between  their  opposing  surfaces  of  perhaps 
one  eighth  of  an  inch. 


58 


THE  RUHMKORFF  COIL. 


Fig.  21. 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  59 

This  space  is  fitted  with  filings  of  nickel. 

The  arrangement  of  the  circuit  is  shown  in  Fig. 
21.  C is  the  coherer  made  as  described  above, 
B a cell  of  dry  battery,  R the  coils  of  the  relay, 
or  an  electric  bell  if  so  desired,  G G the  ground 
connections,  I the  Ruhmkorff  coil,  A A wires  or 
rods  standing  upright  and  parallel.  The  primary 
circuit  of  the  coil  is  not  shown;  it  may  be  the 
ordinary  contact  breaker  or  a telegraph  key. 
D D are  the  balls  of  the  discharger. 

The  air  wires,  or  antennae,  vary  in  length  ac- 
cording to  the  power  of  the  instrument.  For 
experiments  in  a room  or  between  two  rooms,  they 
may  be  either  metal  rods  a few  feet  in  height,  or 
wires  running  upwards  and  attached  to  insulators. 

The  ground  connections  may  be  made  to  a 
water  pipe,  but  if  the  apparatus  is  all  in  one  room, 
a piece  of  copper  wire  run  from  the  coil  terminal 
to  the  coherer  will  be  better. 

The  coherer  plugs  (to  which  the  wires  are  at- 
tached as  shown)  are  now  adjusted  so  that  the 
bell  does  not  ring.  The  coherer,  it  will  be  noticed, 
acts  as  as  ort  of  push  button  of  variable  resistance. 

Upon  snapping  the  coil  contact  breaker,  a spark 
jumps  across  between  D D. 

An  electric  wave  is  thus  liberated  which  travels 
up  the  antenna  on  the  coil  and  is  thrown  off  in 
the  form  of  a ring  gradually  increasing  in  diameter 
until  it  meets  A on  the  coherer. 

Here  it  sends  an  electric  charge  down  this  an- 
tenna through  the  coherer  to  the  ground. 


60 


THE  RUHMKORFF  COIL. 


In  passing  through  C the  charge  rearranges  the 
particles  of  nickel  and  their  combined  resistance 
is  lowered. 

The  battery  B then  is  able  to  send  current 
through  the  nickel  in  the  coherer  and  through  the 
bell  or  relay  coils  R. 

The  bell  or  relay  now  gives  one  stroke. 

Before  another  signal  can  be  sent  the  coherer 
must  be  tapped  so  as  to  shake  up  the  filings. 

In  the  earlier  forms  of  coherers  this  tapping  was 
automatic,  being  accomplished  by  a tapper  on  the 
relay  or  bell  lever  itself. 

A simple  set  as  described  can  be  easily  made 
and  after  adjustment  will  work  over  a consider- 
able distance  depending  upon  the  strength  of  the 
coil,  length  of  the  antennae  and  sensitiveness  of 
the  coherer. 

Tesla  Coils.  The  experiments  with  the  Tesla 
coil  are  most  striking  and  some  of  them  can  be 
performed  in  a certain  degree  with  a coil  as  small 
as  one  giving  an  inch  spark.  The  apparatus  neces- 
sary is  a Tesla  coil,  a condenser  and  a suitable 
discharger.  This  is  in  addition  of  course  to  the 
induction  coil  and  usual  accessories. 

The  Tesla  coil  consists  of  a primary  coil,  a sec- 
ondary coil,  and  a tank  of  oil.  There  is  no  iron 
core  and  the  number  of  turns  of  wire  is  very  much 
smaller  than  in  an  induction  coil  of  the  usual 
construction. 

A Tesla  coil  is  used  in  conjunction  with  a Ruhm- 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  61 

korff  coil  taking  the  secondary  current  from  the 
latter  as  its  primary  current. 

Its  primary  current  is  thus  already  at  a high 
voltage  and  by  means  of  a condenser  and  spark 
gap,  this  current  is  made  to  act  with  great  energy 
in  a series  of  extremely  rapid  impulses. 


D 


Construction  of  Tesla  Coil.  The  construction  of  a 
small  Tesla  coil  does  not  present  very  much 
difficulty.  The  degree  of  finish  or  elaboration 
depends  upon  the  taste  of  the  construction,  an 
efficient  coil  can  be  made  on  the  following  plan: 
In  Fig.  22,  A B are  two  discs  of  hard  wood,  or 
if  possible  of  hard  rubber.  They  are  four  inches 


62 


THE  RUHMKORFF  COIL. 


in  diameter  by  one  quarter  to  one  half  of  an  inch 
thick.  The  figures  are  not  to  scale. 

Six  strips  of  wood  or  hard  rubber  D are  cut, 
seven  inches  long,  three  eighths  of  an  inch  wide 
and  one  quarter  of  an  inch  thick,  and  holes  drilled 
in  the  ends. 

They  are  then  screwed,  with  short  broad  wood 


D 


Fig.  23. 

screws,  to  the  discs  in  such  manner  as  to  form  a 
cage  or  drum,  as  in  the  figure,  which  however 
only  shows  two  strips. 

An  end  view  in  Fig.  23  shows  one  of  the  discs 
and  the  ends  of  the  strips. 

This  cage  is  to  form  a spool  as  it  were  upon 
which  to  wind  the  fine  wire  of  the  secondary  coil. 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  63 


The  mode  of  construction  is  preferable  to  that  using 
an  insulating  tube  because  the  surface  contact  is 
less  and  there  is  less  leakage  between  adjacent 
turns. 

To  make  the  primary  coil,  about  eight  feet  of 
No.  8 B.  & S.  hard  drawn  copper  wire  is  wound 
around  a roller  one  and  one  quarter  inch  in  diam- 
eter. This  can  best  be  done  by  revolving  the 
roller  in  a lathe  but  if  no  lathe  is  accessible,  it 
must  be  done  by  hand. 

In  getting  the  wire  to  lie  snug  a hammer  may 
be  used. 

The  coil  when  finished  is  to  be  pulled  out  into 
a spiral  a trifle  less  than  six  inches  long.  There 
should  be  between  each  turn  a space  more  than 
the  diameter  of  the  wire,  in  fact,  four  turns  to 
the  inch  is  sufficient.  This  will  make  twenty-four 
turns  in  all.  Ends  are  left  for  attachment,  about 
six  inches  long. 

The  discs  and  strips  are  now  taken  apart  pre- 
paratory to  installing  the  primary  coil.  A hole 
is  bored  in  each  disc  three  quarters  of  an  inch 
from  its  centre. 

One  end  of  the  coil  just  made  is  pushed  through 
a hole  in  each  disc  and  the  strips  screwed  in 
place.  If  the  coil  has  been  well  made  the  strength 
of  the  wire  will  keep  it  in  a coil  and  prevent  it 
from  sagging. 

Fig.  22  shows  the  discs  with  the  coil  and  two 
strips  in  place. 

The  strips  being  all  in  place,  the  secondary  coil 


64 


THE  RUHMKORFF  COIL. 


is  wound.  This  consists  of  No.  30  to  36  B.  & S. 
bare  copper  wire. 

It  is  wound  evenly  over  the  strips  as  at  S, 
Fig.  24,  there  being  100  turns  in  all,  which  gives 
an  average  distance  between  turns  of  one  sixteenth 
of  an  inch.  The  secondary  should  not  come  quite 
up  to  the  discs  at  either  end. 

The  first  end  may  be  secured  under  one  of  the 


short  strip-screws,  enough  should  be  left  for  con- 
nection, say  six  inches. 

The  winding  is  best  done  in  a lathe  as  before 
stated,  but  the  turns  are  so  few  comparatively 
speaking  that  no  difficulty  should  be  experienced 
in  winding  by  hand. 

If  holes  have  been  bored  in  the  disc  centre  as, 
temporary  winding  machine  may  be  improvised. 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS  65 


A shaft  of  iron  rod  is  thrust  through  these  holes 
and  turns  in  bearings  made  of  pieces  of  board 
nailed  to  the  sides  of  a box.  One  end  of  the 
shaft  may  be  bent  to  form  a handle  to  rotate 
the  cage.  Mechanical  details  for  accomplishing 
this  work  will  suggest  themselves  to  anyone  at 
all  familiar  with  tools. 

The  primary  and  secondary  coils  are  now  com- 
plete and  ready  for  mounting. 

As  the  coil  is  to  work  immersed  in  oil,  a tank 
must  be  made. 

This  tank,  Fig.  25,  is  in  the  form  of  a box,  all 
parts  of  which  must  be  screwed,  not  nailed  or 
glued  together.  It  should  be  ten  inches  long  by 
six  inches  wide  and  six  inches  deep.  These  are 
inside  measurements.  The  thickness  is  that  most 
convenient.  One  half  inch  of  oak  is  good. 

The  wound  coil  may  rest  in  blocks  of  wood  B B 
as  shown  in  the  figure,  but  the  blocks  must  not  be 
fastened  to  the  box.  If  screws  or  nails  were 
used  they  would  lead  the  current  through  the  sides 
of  the  box  and  set  up  a leakage. 

The  greatest  care  must  be  taken  to  prevent  this 
leakage.  In  all  cases  the  current  will  take  the 
easiest  path,  and  most  troubles  developing  in  this 
form  of  coil  are  to  be  traced  to  a leakage.  The 
current  has  such  high  disruptive  power  that  it 
will  force  its  way  through  the  most  unlikely  places. 

The  best  precaution  against  this  is  to  lead  the 
ends  of  the  coils  as  far  apart  as  possible,  and  not 
trust  to  insulation. 


66 


THE  RUHMKORFF  COIL. 


CL 


CL 


Fig.  25. 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  67 


The  primary  wires  may  be  brought  out  of  holes 
in  the  ends  of  the  box  and  attached  to  binding 
posts  P P as  shown.  In  this  case  the  holes  through 
which  the  wires  pass  must  be  well  caulked  or  the 
oil  will  leak. 

A better  plan  for  coils  to  be  used  with  Ruhmkorff 
coils  of  large  size  is  that  described  for  the  sec- 
ondary coil  wires. 

The  wires  from  the  secondary  coil  are  brought 
out  through  tubes  of  glass  or  hard  rubber.  These 
tubes  must  reach  clear  down  to  the  top  of  the 
coil  and  be  thick. 

They  should  lie  at  an  angle  inclining  away 
from  each  other  so  as  to  separate  the  two  ends  as 
far  as  possible. 

The  utmost  precaution  must  be  taken  to  keep 
the  four  wires  as  far  from  each  other  as  possible. 
Upon  this  being  well  done  depends  much  of  the 
success  of  the  apparatus.  Otherwise  the  current 
will  discharge  from  one  to  the  other,  and  thus 
short  circuit  the  coils. 

The  oil  used  is  that  known  as  transformer  oil, 
procurable  at  most  large  electrical  supply  stores. 
If  not  to  be  had,  heavy  paraffin  oil  will  answer, 
but  it  must  be  absolutely  clean. 

Oil  that  has  been  filtered  through  a water  filter 
will  not  do. 

The  coil  is  to  be  covered  to  a depth  of  one  inch 
with  oil,  and  set  for  a few  hours  in  a warm  place 
to  induce  the  rising  of  air  bubbles. 


G3 


THE  RUHMKORFF  COIL. 


Condenser.  This  should  be  oil  insulated  and 
consists  of  two  thin  metal  plates  three  inches 
square  provided  with  wires  soldered  on  for  con- 
nections. These  wires  must  be  enclosed  in  glass 
or  rubber  tubes  to  prevent  the  spark  jumping 
across  the  plates  outside  of  the  oil. 

The  plates  are  to  be  hung  in  a porcelain  jar 
filled  with  the  same  oil  as  used  in  the  coil  case. 
Or  a similar  wooden  tank  may  be  constructed. 

The  supports  for  the  plates  are  best  made  of 
glass,  porcelain  or  rubber.  If  two  rods  are  used 
resting  across  the  jar,  the  plates  may  be  made 
oblong  and  bent  over  the  rods. 

The  rods  are  made  adjustable  by  means  of  the 
supports,  so  that  the  distance  between  them  may 
be  adjusted,  the  condenser  effect  depending  upon 
this  distance. 

The  Leyden  jar  or  glass-plate  form  of  con- 
denser is  used  by  some  experiments,  but  the  oil 
immersed  type  is  preferable.  However,  the  reader 
may  suit  himself.  A good  plan  is  to  try  both  and 
note  the  difference. 

Discharger.  This  is  similar  to  the  discharger  on 
the  coil  only  it  is  mounted  on  a detached  base. 

It  carries  a brass  ball  one  inch  in  diameter  on 
the  end  of  each  rod.  The  spark  gap  varies  from 
one  eighth  of  an  inch  upwards,  being  adjusted 
until  desired  results  are  obtained. 

The  brass  balls  should  be  well  polished  and 
kept  bright  as  much  of  the  quickness  of  discharge 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  69 

depends  upon  the  nature  of  the  surfaces  between 
which  it  takes  place.  Brass  knobs  similar  to  those 
used  on  the  ends  of  brass  bedsteads  will  answer 
very  well. 

Connections  of  the  Tesla  Coil.  The  connections 
of  the  Tesla  coil  to  the  Ruhmkorff  coil  are  shown 
in  Fig.  26.  A B are  the  secondary  terminals  of 
the  Ruhmkorff  coil  R \ C D are  the  primary  ter- 
minals of  the  Tesla  coil  T \ E F are  the  two  plates 
of  the  condenser,  G H being  the  balls  of  the  dis- 
charger. The  terminals  of  the  secondary  coil  in 
T are  shown  at  J and  K. 

It  will  be  seen  that  the  condenser  is  bridged 
across  the  terminals  of  the  coil  R}  one  plate  then 
being  directly  connected  to  the  primary  of  T, 
and  the  other  plate  being  connected  on  the  top 
of  the  discharger. 

The  condenser  is  charged  directly  from  the 
Ruhmkorff  coil  and  discharges  across  the  air  gap 
when  its  current  is  at  its  maximum  power. 

This  discharge  flows  through  the  primary  of  the 
Tesla  coil  and  acts  in  this  coil  as  the  interrupted 
battery-current  does  in  the  Ruhmkorff  coil. 

As  was  before  stated,  the  quicker  the  break 
at  the  contact  breaker,  the  greater  the  effect  in 
the  secondary  coil.  No  mechanical  break  could 
have  the  abruptness  of  current  breaking  that  is 
produced  by  the  discharge  of  a condenser. 

The  current  flowing  then  through  the  primary 


70 


THE  RUHMKORFF  COIL 


Fig.  26, 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  71 


of  the  Tesla  coil  is  thus  broken  with  the  utmost 
abruptness. 

The  high  voltage  of  the  Ruhmkorff  coil  being 
used  as  a primary  voltage,  induces  a secondary 
voltage  in  the  Tesla  coil  of  vastly  increased  voltage. 

If  there  were  an  iron  core  in  the  Tesla  coil  it 
would  spoil  the  result,  because  it  would  slow  down 
the  breaks,  acting  as  a retardation  upon  them. 

The  smaller  the  condenser,  the  quicker  its  charge 
and  discharge  with  a given  charging  voltage. 

Consequently  by  using  larger  or  smaller  plates, 
or  altering  the  distance  between  them,  the  result 
is  changed  in  the  secondary  of  the  Tesla  coil. 

The  current  takes  an  appreciable  time  to  cease 
owing  to  the  formation  of  an  electric  arc.  This 
arc  carries  the  current  for  an  instant  gradually 
letting  it  die  out. 

Although  this  “ gradually  ” may  mean  only 
the  thousandth  part  of  a second  it  can  never 
equal  the  quickness  of  the  current  cessation  in 
the  condenser  discharge. 

The  relation  between  the  strength  of  an  in- 
duced current  and  the  quickness  with  which  the 
inducing  current  ceases  is  embodied  in  a math- 
ematical formula  to  be  found  in  most  advanced 
work  on  electricity  and  need  not  be  considered 
here. 

Small  Tesla  Coil.  For  those  who  do  not  care 
to  make  the  previously  described  coil  at  first,  a 
simpler  form  may  be  made  as  follows: 


72 


THE  RUHMKORFF  COIL. 


Procure  eight  inches  of  glass  tube  three  fourths 
inch  in  diameter,  and  wind  around  it  one  layer 
of  No.  18  rubber-covered,  copper  wire.  The 
wire  known  as  fixture  wire  will  do  for  experiment. 
This  forms  the  primary  coil. 

A second  glass  tube  is  needed  for  the  secondary 
and  should  be  not  less  than  one  and  one  half  inches 
in  diameter  inside,  but  two  inches  shorter  than 
the  first  tube.  A glass  lamp  chimney  with  par- 
allel sides  will  answer  if  cut  to  length. 

On  this  is  wound  one  layer  of  No.  36  B.  & S. 
double,  silk-covered,  copper  magnet  wire.  This 
forms  the  secondary  coil. 

The  primary  coil  is  to  be  held  centrally  in  the 
secondary  coil  and  both  should  be  supported  inde- 
pendently. 

It  is  best  done  by  using  the  oil  tank  sides  and 
bottom  as  will  be  described. 

A wooden  tank  long  enough  to  hold  the  longer 
tube  is  needed,  one  with  inside  dimensions  of 
eight  and  one  half  inches  long  by  four  inches  wide 
and  four  inches  deep  will  answer. 

The  secondary  coil  may  rest  on  blocks  pro- 
jecting from  the  bottom. 

The  primary  coil  can  then  be  supported  by 
cleats  on  the  inside  ends  of  the  box,  but  it  must 
be  central  in  the  secondary  coil. 

The  details  of  oil  and  connections  will  be  prac- 
tically the  same  as  those  given  for  the  larger  coil. 

The  experiments  that  can  be  performed  with 


WIRELESS  TELEGRAPHY  AND  TESLA  COILS.  73 


Tesla  coil  are  mostly  in  connection  with  luminous 
effects. 

The  discharge  between  rods,  or  balls,  connected 
to  the  secondary  coil,  are  more  like  flames  than 
sparks.  To  better  show  the  brilliancy  of  these 
discharges,  special  devices  can  be  attached  to  the 
discharger  of  the  Tesla  coil. 

One  form  of  discharger  consists  of  two  stout 
wires  running  towards  each  other,  then  bent  up 
at  right  angles  leaving  a space  between  of  an  inch 
or  so. 

Another  is  made  of  two  rings,  one  smaller  than 
the  other.  The  larger  one  may  be  four  inches  in 
diameter  and  the  other  two.  These  are  so  placed 
that  one  lies  concentric  with  the  other  leaving  a 
clear  equal  space  between. 

Although  the  discharges  from  this  form  of  coil 
are  very  striking,  they  are  generally  more  harmless 
than  those  from  the  Ruhmkorff  coil. 

For  X-ray  work,  the  tube  gives  better  penetra- 
tion when  connected  to  the  secondary  of  a dis- 
rupture  coil. 

It  is  of  very  little  use,  however,  in  wireless 
telegraphy,  except  with  special  apparatus,  wThich 
is  of  too  complicated  a nature  to  be  described 
here. 


Price,  25  cts 


jcK  geJEXPERIMENTIISG  WITH 

ITTnddgtiois  Coils 


Containing  practical  directions  for  operating  Induction  Coils  and 
Tesla  Coils ; also  showing  how  to  make  the  apparatus 
needed  for  the  numerous  experiments  described. 


BY 


H.  S.  NORRIE, 

Author  of  44  Induction  Coils  and  Coil  Making.9* 


Fully  Illustrated  with  Original  Drawings, 


5c.  BOOKS. 


ELECTRICITY.  The  study  of,  and  its  laws  for  beginners,  com- 
prising the  laws  of  electric  current  generation  and  flow,  Ohm’s  law, 
galvanism,  magnetism,  induction,  principles  of  dynamos  and  motors, 
wiring,  with  explanations  of  simple  mathematics  as  applied  to  elec- 
trical calculations.  By  N.  H.  Schneider.  With  55  original  illustra- 
tions and  6 tables. 

DRY  BATTERIES.  A practical  handbook  on  the  designing,  fil- 
ling and  finishing  of  dry  batteries,  with  tables,  for  automobiles,  gas 
engine,  medical  and  coil  work,  electric  bells,  alarms,  telephones,  ex- 
periments and  all  purposes  requiring  a first-rate  battery.  Fully  il- 
lustrated with  30  original  drawings. 

ELECTRICAL  CIRCUITS  AND  DIAGRAMS.  Being  a selec- 
tion of  original  up-to-date  and  practical  diagrams  for  installing  an- 
nunciators, alarms,  bells,  electric  gas  lighting,  telephones,  electric 
power  light  and  wiring  circuits,  induction  coils,  gas  engine  igniters, 
dynamos  and  motors,  armature  windings.  By  N.  H.  Schneider. 

ELECTRIC  BELLS  AND  ALARMS.  How  to  install  them.  By 
N.  H.  Schneider.  Including  batteries,  wire  and  wiring,  circuits, 
pushes,  bells,  burglar  alarms,  high  and  low  water  alarms,  fire  alarms, 
thermostats,  annnuciators,  and  the  locating  and  remedying  of  faults. 
With  56  original  diagrams. 

MODERN  PRIMARY  BATTERIES.  Their  construction,  use 

and  maintenance,  including  batteries  for  telephones,  telegraphs, 
motors,  electric  lights,  induction  coils,  and  for  all  experimental 
work.  By  N.  H.  Schneider.  94  pages,  55  illustrations.  The  best  and 
latest  American  book  on  the  subject. 

EXPERIMENTING  WITH  INDUCTION  COILS.  H.  S.  Norrie, 

author  of  “ Induction  Coils  and  Coil  Making.**  A most  instructive 
little  book,  full  of  practical  and  interesting  experiments,  fully  ex- 
plained in  plain  language  with  numerous  hints  and  suggestions  for 
evening  entertainments.  Arranged  under  the  following  headings: 
Introduction;  The  Handling  of  Ruhmkorff  Coil;  Experiments  with 
Sparks;  Effects  in  the  Vacuum;  Induction  and  Wireless  Telegraphy. 
With  36  original  illustrations.  [In  the  press] 

SMALL  ACCUMULATORS.  How  made  and  used,  by  P.  Mar- 
shall. Giving  full  descriptions  how  to  make  all  the  parts,  assemble 
them,  charge  the  cells  and  run  them,  with  examples  of  their  practi- 
cal application.  Useful  receipts  and  memoranda  and  a glossary  of 
technical  terms.  80  pages,  40  illustrations,  paper. 

ELECTRIC  GAS  LIGHTING.  How  to  install  Electric  gas  ignit- 
ing apparatus  including  the  jump  spark  and  multiple  systems  for  all 
purposes.  Also  the  care  and  selection  of  suitable  batteries,  wiring 
and" repairs,  by  H.  3.  Norrie.  ioi  pages,  57  illustrations,  paper. 


c.  BOOKS 


MAKING  WIRELESS  OUTFITS.  By  Newton  Harrison,  E.E. 
A concise  and  simple  explanation  on  the  construction  and  use  of 
simple  and  inexpensive  wireless  equipments,  for  sending  and  re- 
ceiving up  to  100  miles,  giving  full  details  and  drawings  of  apparatus, 
diagrams  of  circuits  and  tables.  Including  the  Morse  and  Con- 
tinental Codes.  61  pages,  27  illustrations. 

CIRCUITS  AND  DIAGRAMS.  Part  2.  By  Norman  IT. 
Schneider.  Alternating  Current  Generators  and  Motors:  Single 

Phase  and  Polyphase  Transformers:  Alternating  Current  and  Direct 
Current  Motor  Starters  and  Reversers:  Arc  Generators  and  Cir- 
cuits: Switch- Wiring:  Storage  Battery : Meter  Connections:  etc. 

etc.  69  original  drawings,  with  full  explanations. 

ALTERNATING  CURRENTS  SIMPLY  EXPLAINED.  An  Ele- 
mentary Handbook  on  Alternating  Current  Generators,  Trans- 
formers, and  Motors.  By  Alfred  W.  Marshall.  This  book  is 
written  for  those  who  desire  elementary  information  about  Alter- 
nating electric  currents,  simply  written  and  yet  intensely  interest- 
ing. Contents  of  Chapters: — 1.  What  an  Alternating  Current  is. 
2.  How  Alternating  Currents  are  Produced.  3.  How  Alternating 
Currents  are  Measured. ' 4.  Transformers  and  Choking  Coils.  5. 
Alternating  Current  Motors.  6.  Rotary  Converters.  7.  Rectifiers. 
82  pages,  32  illustrations. 

INDUCTION  COILS.  How  to  Make  and  Use  Them,  by  P. 
Marshall.  New  edition  revised  and  enlarged  by  K.  Stoye.  A 
practical  handbook  on  the  construction  and  use  of  medical  and 
sparking  coils  for  wireless  telegraphy,  gas  engines,  automobiles, 
gas  lighting,  X-rays,  and  all  other  purposes.  With  complete 
tables  of  windings  for  coils  giving  | in.  spark  up  to  12  in.  sparks. 
With  full  description  for  the  construction  of  mercury  interrupters. 
76  pages,  35  illustrations. 

SIMPLE  EXPERIMENTS  IN  STATIC  ELECTRICITY.  By  P.  C. 

Bull,  M.A.  Contents  of  Chapters: — 1.  Production  of  electricity 
by  various  means.  Viz. : friction,  heat,  pressure,  chemical  action, 
etc.  2.  Electrical  attraction,  repulsion,  and  distribution.  3. 
Induction.  4.  Leyden  jars  and  other  condensers.  5.  Mechanical, 
chemical  and  heating  effects.  6.  Luminous  effects.  7.  Miscel- 
laneous experiments.  Being  a series  of  instructive  and  entertaining 
electrical  experiments.  72  pages,  51  illustrations. 

THE  MAGNETO  TELEPHONE.  Its  construction,  fitting  up  and 
use,  by  Norman  Hughes.  Giving  full  particulars  for  planning 
out  a short  line,  putting  up  the  insulators,  stringing  wires, ' con- 
necting instruments,  with  suitable  batteries.  80  pages,  23  illus- 
trations, including  a number  of  diagrams  of  circuits. 


25c.  BOOKS 

WIRELESS  TELEPHONE  CONSTRUCTION.  By  Newton 
Harrison.  A comprehensive  explanation  of  the  making  of  a 
Wireless  Telephone  Equipment.  Both  the  transmitting  and  re- 
ceiving stations  fully  explained  with  details  of  construction  suffi- 
cient to  give  an  intelligent  reader  a good  start  in  building  a Wireless 
Telephone  system  and  in  operating  it.  74  pages  and  43  illustrations. 

THE  WIMSHURST  MACHINE.  HOW  TO  MAKE  AND  USE  IT. 

A practical  handbook  on  the  construction  and  working  of  Wimshurst 
machines,  including  radiography  and  wireless  telegraphy  and  other 
static  electrical  apparatus.  By  A.  W.  Marshall.  Second  edition, 
revised  and  enlarged.  Containing  a number  of  sectional  drawings 
and  details  to  scale.  112  pages,  fully  illustrated. 

SMALL  ELECTRICAL  MEASURING  INSTRUMENTS.  How  to 

Make  and  Use  Them.  By  Percival  Marshall.  Contents  of 
Chapters: — 1.  Instruments  for  testing  the  presecne  of  an  electric 
current,  detectors,  galvanometers.  2.  Instruments  for  measuring 
the  pressure  or  quantity  of  an  electric  current,  amperemeters; 
voltmeters.  3.  Instruments  for  measuring  electrical  resistance, 
wheatstone  bridge.  4.  Instruments  for  measuring  static  elec- 
tricity. 5.  Practical  details  for  construction.  6.  The  principles 
upon  which  electrical  measuring  instruments  work.  7.  How  to  use 
electrical  measuring  instruments.  8.  How  to  choose  electrical 
measuring  instruments.  90  pages,  59  illustrations. 

INVENTIONS.  How  to  Protect,  Sell  and  Buy  Them.  By 
Frederic  B.  Wright.  Counsellor  in  Patent  Causes.  This  book 
is  especially  written  for  the  use  of  Inventors,  instructing  them  how 
to  place  their  inventions  before  an  Attorney  clearly;  the  rights  given 
them  under  the  Law,  Patent  specifications,  Legal  forms,  and  the 
many  points  necessary  for  an  Inventor  to  know  to  protect  himself 
under  the  American  Laws.  The  most  practical  and  clearly  written 
American  book  on  this  subject,  especially  intended  for  the  un- 
initiated. 114  pages,  and  1 sample  pattern  drawing. 

UNIVERSAL  TIME  CARD  MODEL.  By  setting  to  the  desired 
hour  at  any  one  place  the  movable  model  will  show  at  a glance  the 
actual  time  of  all  the  other  places  in  the  world.  Printed  on  stiff 
card  in  two  colors,  size  7 in.  by  9 in. 

HOW  TO  BUILD  A 20  FT.  BIPLANE  GLIDING  MACHINE, 

that  will  carry  a man.  By  A.  P.  Morgan.  A practical  handbook 
on  its  construction  and  management.  Enabling  an  intelligent 
reader  to  make  his  first  step  in  the  field  of  aviation  with  a compre- 
hensive understanding  of  some  of  the  principles  involved.  Fully 
illustrated  with  detailed  drawings. 


